1. Field of the Invention
The present invention relates to a sealing glass composition for hermetically sealing two members and, in particular, to such a composition for use in production of electronic parts or devices such as semiconductor devices, fluorescent character display tubes, and others.
2. Description of the Prior Art
Generally speaking, the sealing glass composition is a powdery mixture comprising solder glass powder of a low melting temperature and refractory filler powder. As the solder glass, PbO-B.sub.2 O.sub.3 glass has been used which consists essentially of PbO 40-90 wt % and B.sub.2 O.sub.3 8-15wt %. Another solder glass is a PbO-ZnO-B.sub.2 O.sub.3 glass which consists essentially of PbO 70-85 wt %, ZnO 0.5-7 wt %, and B.sub.2 O.sub.3 7-15 wt %. These solder glasses generally have thermal expansion coefficients of about 90.times.10.sup.-7 /.degree.C. or more which is higher than that of members of be sealed or bonded. Therefore, the filler powder is used for matching the sealing glass composition with the sealed members in the thermal expansion coefficient. Conventionally, zircon, willemite, cordierite, .beta.-eucryptite, tin material (or tin oxide), and/or lead titanate are used as the refractory filler powder.
Among those various refractory fillers, lead titanate (PbTiO.sub.3) is conveniently used, because use of a comparatively small amount of the lead titanate can provide a preferable sealing composition having a thermal expansion coefficient matching the sealed member.
However, in order to lower the thermal expansion coefficient of the sealing glass composition to match that of the sealed members, it is required to use lead titanate powder of a comparatively large particle size, for example, more than 15 .mu.m. But use of lead titanate powder having such an increased particular size makes it difficult to deposit, by screen printing, a paste of the glass composition dissolved in a vehicle onto members to be bonded. Further, after sealing, microcracks are apt to cause in glass phase around the lead titanate crystals.
In order to provide a glass sealing composition using PbO-TiO.sub.2 crystals for the filler wherein a lower thermal expansion coefficient can be realized and microcrack is not caused even if the filler has a small average particle size of, for example, 15 .mu.m or less and is used by a small amount, U.S. Pat. No. 4,710,479 (Reference 1) issued to Yamanaka et al and assigned to Nippon Electric Glass Company, Limited discloses to use the filler which consists essentially of a lead calcium titanate represented by the following chemical formula (Pb.sub.1-m Ca.sub.m)TiO.sub.3 where 0&lt;m.ltoreq.0.40.
Although the glass composition has a reduced thermal expansion and a reduced number of microcracks after sealing, a small number of microcracks is still present in the glass after sealed. This is not desirable for the sealing glass.